1. Field of the Invention
The present invention relates to a flat cathode-ray tube used for such devices as the picture tube and the image display unit for video equipment.
2. Description of Related Art
FIG. 1 is a schematic plan sectional view showing a configuration of a conventional flat cathode-ray tube. In FIG. 1, numeral 7 designates a flat metal housing including a front metal case 7a and a rear metal case 7b. The front side of the front metal case 7a is open, and has a screen glass 4 formed with a phosphor layer 5 sealed from the front side thereof through crystallized frit glass (or a low-melting-point glass, hereinafter referred to as "the frit glass") 15. The front metal case 7a and the screen glass 4 are sealed by glass fusion in some applications. The metal case 7 has built therein an electron beam forming unit as a kind of electron gun including a cathode 1 making up an electron beam source, electron beam extraction means 2 for extracting an electron beam from the cathode 1 and electron beam control means 3 for controlling the passage of the electron beams extracted by the electron beam extraction means 2 with a plurality of electrode plates.
The cathode 1 and the electron beam extraction means 2 are fixed in that order inside of the rear metal case 7b. The electron beam control means 3 has springs 12, 12 mounted at the ends thereof and is suspended thereby, which springs 12, 12 are detachably supported on stud pins 11, 11 of ceramics erected from the side inner wall of the front metal case 7a.
The metal case 7 includes a front metal case 7a with electron beam control means 3 mounted thereon and a rear metal case 7b fixed with a cathode 1 and electron beam extraction means 2 coupled and sealed in opposed relationship to each other. Further, an exhaust pipe 13 for exhausting the interior of the metal case 7 to an ultrahigh vacuum state (10.sup.-5 Pa or less) is arranged on the rear metal case 7b.
Explanation will be made about the operation of the flat cathode-ray tube configured as described above. Upon application of a predetermined voltage to the electron beam extraction means 2 with the cathode 1 maintained at a predetermined potential, an electron beam is extracted from the cathode 1. The passage of the electron beam is controlled by applying a control signal to the electron beam control means 3. When the electron beam is thus correctly impinged on the phosphor layer 5, an image is reproduced. In recent years, as described above, the trend is toward a metal, instead of glass, case employed in order to alleviate the increased weight with the increase in size.
In this flat cathode-ray tube, in order to couple strongly the screen glass 4 and the front metal case 7a to each other through frit glass 15, as shown in FIG. 2, a Cr oxide film (Cr.sub.2 O.sub.3) 20 of a few .mu.m thick is required to be formed as a preliminary treatment of the metal material (front metal case 7a). FIG. 3 is an enlarged sectional view showing the coupling portion between the front metal case 7a formed with the Cr oxide film 20 and the screen glass 4 through the frit glass 15.
The oxide film such as Cr oxide film 20, is formed in various ways. Considering the film minuteness and adherence to metal, the wet-hydrogen environment high-temperature oxidation method is considered superior in general. A stainless steel material (SUS430), for example, is known to be formed with a 3-.mu.m oxide film after the process of 1000.degree. C..times.about 6 hours. Coupling between the oxide film formed on the metal surface and the frit glass, however, is not considered to have a sufficient coupling strength against the vacuum stress, and this coupling strength is insufficient as a structure of a vacuum case.
It is obvious, on the other hand, that the heating of a metal for long time at high temperatures is a cause of thermal deformation and has an adverse effect on the mechanical properties thereof. As it is known, an early roughening of crystalline particle of some materials leads to brittleness. Also, the heating reduces the flatness of the coupling surface, thereby uniform coupling being made difficult. The problem is therefore that dimensional variations are likely to occur after coupling.